5. PROJECT SUMMARY ? PROJECT 2 The goal of this project is to validate noninvasive methods of measuring whole body phenylalanine (Phe) metabolism rates that are not confounded by dietary Phe intake in individuals with phenylalanine hydroxylase (PAH) deficiency. Future clinical trials of liver-directed gene therapy for the treatment of PAH deficiency will require the use of such methods to measure treatment efficacy. We propose to evaluate two different methods of measuring Phe metabolism: 1) measurement of 13CO2 enrichment in breath over time following oral administration of 1-13C-L-phenylalanine, and 2) measurement of plasma Phe clearance and plasma tyrosine production following oral unlabeled Phe challenge in a fasting state. These measures will be repeated over time in the same cohort of study subjects with full or partial PAH deficiency in order to assess the sensitivity and test-retest variability of the methods. This project will assess the sensitivity and stability of the proposed testing methods in measuring Phe metabolism non-invasively and prepare a clinical trial ready protocol for future use.